Huge Tunguska Explosion Remains Mysterious 100 Years Later

A full century after the mysterious Tunguska explosion in Siberia leveled an area nearly the size of Tokyo, debate continues over what caused it.

Many questions remain as to what crashed into the Earth from above -- how big it was and what it was made of. Some question whether it even came from space at all, or whether it erupted from the ground instead.

And there is always speculation that it was caused by a UFO or famed inventor Nikola Tesla's "death ray."